In telephone circuits of the one-chip type which serve multiple functions normally performed by discrete integrated circuits such as modern speech, ring, and tone circuits--the system common point of reference, both physically and electrically, is an individual one coinciding with the substrate of semiconductor material whereto the whole one-chip circuit is integrated.
In a telephone subscriber system, the telephone subscriber speech circuit and ring circuit are both coupled to, and energized through, the telephone line. During the ring phase, however, any coupling of the two circuits together must be prevented, as by disconnecting the speech circuit through switches, whereas during the speech phase, the ring circuit must be disabled. Current telephone company standards, moreover, call for the ring circuit to be DC uncoupled from the telephone line for any line polarity. This requires, as is well recognized by the skilled ones in the art, either the provision of a dual RC network, or the inclusion of a traditional electric switch which can be readily integrated. Understandably, the last-mentioned solution is the one which would normally be utilized wherever the telephone circuits involved are of the one-chip type.
In a telephone subscriber system, the speech phase is initiated by connecting the telephone line (DC connection) to the telephone subscriber speech circuit as the handset is picked up or, in more sophisticated designs, on a pulse being issued from a keyboard. But, if one wishes to connect the line to the telephone subscriber speech circuit without picking up the handset and without acting physically on the telephone apparatus as by keying in--operation from an external electric control being preferred instead--problems may arise from the line polarity being undefined and the line's own reference potential being other than the reference potential of the one-chip circuit, i.e., its substrate. In fact, an input terminal of the device--namely that connected to the terminal of the polarity bridge, which would usually be a diode bridge enabling the speech circuit to be coupled to the line--might take a negative potential with respect to the system reference, i.e., the device substrate.
To obviate the difficulties brought about by the different electrical references, it has been common practice to employ in most cases either electromechanical devices or opto-electronic components, which are effective but cost-intensive.